The General Packet Radio Service (GPRS) standard provides a compatibility standard for cellular mobile telecommunications systems. The GPRS standard ensures that a mobile station (MS) operating in a GPRS system can obtain communication services when operating in a system manufactured according to the standard. To ensure compatibility, radio system parameters and call processing procedures are specified by the standard, including call processing steps that are executed by an MS and a network in order to provide for cell reselection.
FIG. 1 is a block diagram of an exemplary GPRS communication system 100 of the prior art. Communication system 100 includes multiple base transceiver stations (BTSs) 110, 112 that are each coupled to a Base Station Controller (BSC) 114. Each BTS 110, 112 provides communication services to a respective coverage area, or cell, serviced by the BTS. BSC 114 is further coupled to a Packet Control Unit (PCU) 116. PCU 116 is coupled to a Serving GPRS Support Node (SGSN) 118 that is, in turn, coupled to a Gateway GPRS Support Node (GGSN) 120, and via the GGSN, to an external network 130. BTSs 110 and 112, BSC 114, PCU 116, SGSN 118, and GGSN 120 are collectively referred to as a communication system network 122. Communication system 100 further includes an MS 102 that resides in a first cell and is provided communication services by a BTS 110 serving that cell. MS 102 and serving BTS 110 communicate via an air interface 104 comprising a downlink 105 and a uplink 106. Downlink 105 comprises multiple logical channels, including a broadcast channel, at least one traffic channel, and at least one control channel. Uplink 106 also comprises multiple logical channels, including an access channel, at least one traffic channel, and at least one control channel.
When MS 102 activates in communication system 100, the MS is conveyed a Neighbor List by the serving cell, that is, the cell associated serving BTS 110. Typically, the Neighbor List includes a list of multiple broadcast channels, typically 32 broadcast channels, associated with corresponding neighbor cells, such as a neighbor cell associated with BTS 112. MS 102 then monitors the listed broadcast channels.
In a typical GPRS communication system such as communication system 100, when MS 102 activates in, or roams into, the cell serviced by serving BTS 110, the MS is further is informed, via a Packet System Information (PSI) message transmitted by the BTS, of a cell reselection mode supported by the serving cell. In a GPRS communication system, the cell reselection mode may be any one of three modes. In a first cell reselection mode, NC0 (Network Control Order 0), cell reselection is autonomously performed by an MS based on a strength of a received downlink signal associated with the serving cell, that is, of a signal received by MS 102 from BTS 110 via downlink 105. In a second cell reselection mode, NC 1, cell reselection is autonomously performed by an MS based on a received signal strength of a signal received via a broadcast channel of the Neighbor List, such as a signal received by MS 102 from BTS 112 via a downlink 108 associated with BTS 112. In cell reselection mode NC1, the MS is further required to send measurement reports to the network. In a third cell reselection mode, NC2, cell reselection is controlled by a network, such as network 122. In the third cell reselection mode, an MS sends Packet Measurement Reports (PMRs) to the network, which PMRs include received signal strength measurements associated with the serving cell, such as the cell associated with serving BTS 110, and with multiple, typically six, broadcast channels of the Neighbor List. Based on the received PMRs, network 122 may then instruct MS 102 to perform a cell reselection.
In GPRS communication systems, the cell reselection mode supported by a cell is uniformly applied throughout the cell. That is, all MSs serviced by the cell, that is, by the BTS serving the cell, must use a same, fixed cell reselection mode. A result of the uniform application of a cell reselection procedure is a sub-optimal execution of cell reselections and unnecessarily congested air interfaces. For example, in cell reselection modes NC0 and NC1, an MS such as MS 102 is not aware of congestion level of a cell selected by the MS as a target cell. As a result, in cell reselection modes NC0 and NC1, an MS may request a handoff to a congested target cell with a result that the MS and other users actively engaged in a communication in the target cell experience degradation in a quality of their communications. This result is particularly undesirable when the MS may have been experiencing acceptable conditions in its current cell. By way of another example, cell reselection modes NC1 and NC2 require an MS, such as MS 102, to periodically transmit measurement reports to a network, such as network 122. When the channel conditions experienced by MS 102 are consistently of acceptable quality, the periodic measurement reports transmitted by the MS are, in essence, redundant and are of no use to the network. As a result, when the cell serviced by a BTS, such as BTS 110, includes a large number of MSs, air interface 104, and in particular uplink 106, may become congested with redundant measurement reports that are of little, if any, value to the network. This is of particular importance in light of the fact that over-the-air bandwidth is often a constraining factor in system performance.
Therefore, a need exists for a method and apparatus that optimizes an application of cell reselection modes in a cell.